Atom-thick carbon sheets set new strength record

The carbon supermaterial graphene is already known for its exotic electronic properties. Now two studies suggest that the material is also one of the strongest, most elastic and stiffest materials known to science.

Graphene crystals are atom-thick sheets of carbon atoms connected together in hexagons, like chicken wire.

Graphene flakes are produced every time we put pencil to paper - the graphite in pencils is simply a 3D structure comprising multiple stacked layers of graphene. And yet graphene was only isolated for the first time in 2004.

In the graphene "gold rush" since then, scientists have scrambled to uncover the material's properties and discover potential applications. The large surface-to-volume ratio and high conductivity already suggest uses in ultra-small electronics.

Now, researchers have discovered that graphene has remarkable mechanical properties too. Changgu Lee and Xiaoding Wei at Columbia University, New York, took flakes of graphene 10 to 20 micrometers in diameter and laid them across a silicon wafer patterned with holes just 1 to 1.5 micrometers in diameter, like a microscopic muffin tray.

'Off-the-chart'

The graphene above the tiny holes was unsupported, and Lee and Wei poked at these with the diamond tip of an atomic force microscope to see how readily the graphene deformed and ruptured.

They found that the graphene could be pushed downwards by 100 nanometres with a force of up to 2.9 micronewtons before rupturing. The researchers estimate that graphene has a breaking strength of 55 newtons per metre.

"As a way of visualising the force needed to break the membranes, imagine trying to puncture a sheet of graphene that is as thick as ordinary plastic food wrap - typically 100 micrometers thick," says James Hone, head of the laboratory at Columbia in which Lee studies. "It would require a force of over 20,000 newtons, equivalent to the weight of a 2000 kilogram car."

That strength puts graphene literally "off the chart" of the strongest materials measured, Hone says. "These measurements constitute a benchmark of strength that a macroscopic system will never achieve, but can hope to approach," he says.

In separate work, Tim Booth and Peter Blake at the University of Manchester, UK, are well on the way to bringing atomically perfect graphene out of the nanoscopic and into to the macroscopic world. Their team has patented a new method to produce free-standing graphene flakes up to 100 micrometers in diameter.

Sticky problem

Researchers use sticky tape to remove tiny flakes of graphene from graphite, and as their technique improves they are beginning to produce larger and larger flakes. The flakes can be picked off the sticky tape manually or the tape can be dissolved away with acetone.

A problem with this method is that the sticky tape picks up flakes of multi-layered graphite at the same time. Finding the graphene is like searching for a needle in a haystack.

The key is to place all of the flakes on a silicon wafer where the properties of the graphene make it easy to spot under an optical microscope.

However, graphene flakes bond to the silicon and are easily damaged when the scientists try to remove them. One solution is to use aggressive chemicals such as hydrofluoric acid to eat away the silicon and free up the graphene but this tends to chemically contaminate the graphene and alter its properties.

Now, Booth and Blake have realised that acrylic glass (PMMA) has the same optical properties as silica and can also highlight graphene flakes. It however easily dissolves away in acetone, a less aggressive chemical that doesn't alter graphene. Using their technique, Booth and Blake can easily isolate large crystals.

'Science-fiction applications'

"We are limited only by the size of graphene flakes available," says Booth. "There is no reason that the method will not scale up to much larger flakes."

Using these flakes, Booth and Blake have also found that graphene is extraordinarily stiff. A crystal supported on just one side extends nearly 10 micrometers without any support - equivalent to an unsupported sheet of paper 100 metres in length. It had previously been assumed that graphene would curl up if left unsupported.

Graphene could be added to polymers to form super-strength composites, Booth says. "However, it is likely the most interesting applications will result from a unique combination of graphene's properties: transparency, electronic structure, stiffness, thermal conductivity," he says. "That could help achieve science-fiction applications."

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